Cracking and Crumbling: Exploring Mechanisms of Dike Emplacement

Cracking and Crumbling: Exploring Mechanisms of Dike Emplacement

Phillip G. Resor

Wesleyan University

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Type of Activity: Problem Set/Case Study

Students explore the shape of basaltic dikes intruded into shale near Shiprock, New Mexico using published maps and a simple mechanical model. Students get practice in making quantitative measurements, evaluating models, and generating alternative hypotheses.

Context

Type and level of course in which I (plan to) use this activity or assignment: I plan to use the activity in an undergraduate structural geology course for majors. The activity was originally developed by David Pollard for a graduate-level fracture mechanics class. The activity can be scaled to different levels by adjusting the level at which the mechanical model is explored.

Skills and concepts that students must have mastered before beginning the activity: Before beginning the activity students should have mastered concepts of stress, strain, constitutive laws (specifically elasticity), and rock failure. The activity can be integrated into a discussion of rock strength and fracture.

How the activity is situated in my course:The activity is designed as a stand alone activity that may be completed in one or two problem sets.

Goals of the Activity or Assignment

Content/concepts goals for this activity: strength of rocks, fracture mechanics, application of mechanical models to real-world problems, mechanisms of dike emplacement.

Higher order thinking skills goals for this activity: critical evaluation of models and formulation of hypotheses.

Other skills goals for this activity: collection and evaluation of quantitative data from geologic mapping.

Description

This case study is based on exercises developed by Dave Pollard and uses images and mapping from Delaney and Pollard (1981). Students explore mechanisms of dike emplacement by measuring dike thicknesses along two segments of a dike intruded into shale near Ship Rock, New Mexico and mapped by Paul Delaney. Students then compare the observed dike profiles to a mechanical model – the solution for a crack filled with magma under uniform pressure. Using the equation for opening of the crack students estimate the driving stress (difference between the magma pressure and remote principal stress acting perpendicular to the dike plane) using several different estimates of the rock stiffness parameters. Students are then asked to observe how well their best-fit model fits the two dike profiles and to investigate the map further and hypothesize why the observed widths may deviate from the simple model. Deviations from the mechanical model can lead to discussions on mechanical interaction as well as brecciation and stoping.

Evaluation

Student performance can be assessed by grading a written report with graphs and supporting discussion.

Documentation

The following documentation is still in its developmental stages. I hope to provide the required background information and appropriate maps and images so that you could develop your own handouts, activities, etc. Enough of the geologic map from Delaney and Pollard (1981) is reproduced here so that you can complete the exercise, but a copy of the original map allows for greater appreciation of the entire segmented dike. Much of the supporting material will also be published in a textbook by Pollard and Fletcher, Fundamentals of Structural Geology to be published later this year (2004).

Background

Students should read Delaney and Pollard (1981) sections on Geological and Physical Setting and Mapping Methods and Precision of Data before attempting the lab. This reading provides background on the geologic setting and the methods used to create the geological map. Students should identify and define any terms they are not familiar with.

The northeast dike is a member of a set of minette dikes that radiate from the Ship Rock, a volcanic neck made up of tuff breccia. Delaney and Pollard (1981) estimate that the present-day land surface was between 750 and 2500 m depth when the dikes were intruded based respectively on arguments from volcanic structures and regional stratigraphy. The dikes are presently exposed within the Mancos Shale.

The northeast dike is about 3 km long and is made up of approximately 35 segments. Delaney and Pollard (1981) mapped the entire dike in detail using low-altitude vertical aerial photographs. They used the central portions of the photographs to minimize camera distortion and scaled the photos using field measurements. The photos and maps below show Ship Rock, the northeast dike, and portions of the detailed geologic map. Students can use these maps to create their own data set of dike width profiles for several segments of the dike and then compare their measured profile to the predictions of the model for a pressurized crack.

Aerial Photograph of the northeast dike (Photo D. Pollard)

Paul Delaney on the northeast dike with

Ship Rock in the background (Photo D. Pollard)

Mechanical Model – Opening of a crack filled with a uniform-pressure fluid (magma)

The relative displacement of the walls of a two-dimensional (infinite in the out-of-plane direction) pressurized crack in an isotropic homogeneous linear-elastic solid that has only opening displacements (no shear) is described by the following equation:

where

ΔuI is the width of the dike (opening mode displacement)

ΔσI is the difference between the magma pressure and the least compressive regional stress (driving stress)

μ is the shear modulus

ν is the Poisson ratio

a is the dike half-length

x2 is the distance from the center of the dike

The derivation of this equation is likely beyond most undergraduate structure courses. For the details see Pollard and Segall (1987). The basic form of the equation, however, can be explored both geometrically (can you rearrange to get the equation of an ellipse?) and mechanically – describing the relationship between stress and displacements using a constitutive law (elastic rock properties).

The detailed geologic map (below with close-ups on the following page) of the northeast dike at Ship Rock provides an excellent data set for exploring how well this model fits the intrusion of dikes into shale.

Geologic map of a portion of the northeast dike (Delaney and Pollard 1981)

Measured rock stiffness parameters

Rock Type / Lab or field? / μ (MPa) / ν / source
Clastic sedimentary / Lab / 3,200-47,000 / 0.04-0.29 / Birch, 1966
Black shale / Lab / 5-115 / Hendron et al., 1970
Sandstone / Field / 400-1700 / Dixon, 1970
Siltstone / Field / 20-60 / Dixon, 1970

Potential Questions for students to explore include:

Field data collection: Students can “collect” their own data by measuring the thickness profiles of several dike segments from the maps of Delaney and Pollard.

• What needs to be done to the aerial photos before they can be used for accurate mapping?

Remove distortion and determine photo scale (orthorectification, georeferncing)

• What coordinate system would seem most natural for comparing the data you measure to the model equation?

The equation as written uses distance from the dike center, so a coordinate system with the origin at the dike center and the x-axis parallel to the dike will make for the easiest comparison.

• How precise do you think your measurements are?

Mechanical modeling: Students can then find the best-fit elastic model using rock stiffness parameters from the literature and adjusting the driving stress to get a visual best-fit.

• Explore the range of reasonable rock stiffness parameters. Find the best-fit driving stress by trial and error. What are the implications of stiffer surrounding rocks for the driving stress? How does your estimate compare to the estimate Delaney and Pollard made for the lithostatic stress?
• How well does the model fit, qualitatively?
• Do similar parameters yield reasonable fits to different segments (i.e. 12 and 16)?
• What are some possible explanations for areas where the model does not appear to fit well?

Have students look at the maps again and identify the areas that do not fit the model well. Significant deviations occur in areas of brecciation (see segment 12) and near the tips where dikes overlap each other and are likely to be interacting mechanically

• Mechanical interaction can be discussed conceptually: What might the effect of one dike pushing open be on a neighboring dike?
• For upper-level students the mechanical interaction can be explored using modeling packages (e.g. Poly3D: or even having the students code their own boundary element code for 2 interacting cracks.

Dike Intrusion: Questions for discussion.

• What can we learn about dike intrusion from the success or failure of the mechanical model?
• Propose a model for how dikes may intrude and then evolve into volcanic conduits.

A.

B.

C.

Explanation for geologic map (A). Aerial photo with field mapping and geologic map for segments 16-17 (B) and 12-15 (C).

Examples of comparison between the measured dike thickness (gray points) and elastic model (black dashed line) for segments 16 and 12. The model curves use the same rock stiffness parameters.

References and Further Reading

Bürgmann, R., Pollard, D.D., and Martel, S.J., 1994, Slip distributions on faults: effects of stress gradients, inelastic deformation, heterogeneous host-rock stiffness, and fault interaction: Journal of Structural Geology, v. 16, p. 1675-1690.

Delaney, P. and Pollard, D.D., 1981, Deformation of host rocks and flow of magma during growth of minette dikes and breccia-bearing intrusions near Ship Rock, New Mexico: U.S. Geological Survey Professional Paper, 1202, 61 p.

Pollard, D.D. and Muller, O.H., 1976, The effect of gradients in regional stress and magma pressure on the form of sheet intrusions in cross section: Journal Geophysical Research, v. 81, p. 975-984.

Pollard, D.D. and Segall, P., 1987, Theoretical displacements and stresses near fractures in rock: with applications to faults, joints, veins, dikes, and solution surfaces, in Fracture Mechanics of Rock, B. K. Atkinson. London., Academic Press: 277-349.